Secret signaling system



pi 3, 1946. E, D, COOK 2,399,469

SECRET SIGNALING SYSTEM Filed July 51, 1940 2 sheets-sheet 2 FIQZ.

FREQUENCY ArIPLITuoE Powe R MoDuLAToR ANPI-FER rIoDuLA-ron AMPLIFIER FREQUE NCY OSCILLATOR HODULATOR AMPLIFIER 6 TO SIGNAL SOURCE AMPLIFIER AND HARMDNIC GENERATOR /I /l oscILLATcR Enno An. MIXER AN coNvEnIjeR I InIrER coNvERTER oETEcToR FILTER AMPLIFIER FREQUENCY MODULATOR AMPLIFIER 36 ArIPLIPIER oscILLAToR OSCILLAT OR` RECTIFIER 87 AMPLIFIER AND HARMONIC GENERATOR 86 FILTER Inventor: Ellsworth D. Cook,

I-I'IS Attorney.

Patented Apr. 30, 1946 SECRET SIGNALING SYSTEM Ellsworth D. Cook, Scotia, eral' Electric Company,

York

N. Y., assignor to Gena corporation of New Application July 31, 1940, .Serial.No.v 348,758

(Cl. Z50- 6) 12 Claims.

My invention relates to radioA signaling systems,

particularly to secret signaling systems, and its` general object is. to provide signaling systems in whichk any desired high degree of secrecy is obtained by means including frequency modulations of. a carrier wave.

In accordance with the present invention this object is attained by the provision of a secret signaling system in which two frequency modulations of the carrier wave are employed in succession, one modulation being in accordance with a` desired signal and one in accordance with a distorting signal.

The novel features which are considered to be characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by taken in connection with the accompanying drawings in which Fig. 1 is a diagrammatic representation of a secret signaling system embodying my invention, Fig. la illustrates diagrammatically a construction detail of thesystem shown in Fig. l, and Fig. 2 represents a secret signaling system based upon the system of Fig. 1.

Referring to Fig. l, the numerals I and 2 des,- ignate respectively a transmitter and a receiver for a secrecy signaling system. In the transmitter I a source of oscillations indicated by the numeral 3 is provided for generating a carrierv wave of suitable frequency. Numeral 4'designates an antenna from which the carrier is transmitted after modulation by desired signals which are distorted in a manner to be explained hereinafter.

In series between oscillator 3 and antenna 4 are connected a frequency modulator 5 prefer ably including means (not shown) to adjust the frequency shift thereof, a, linear amplifier B, a second frequency modulator 1 also including means to adjust the frequency shift thereof, a second linear amplifier 8, an amplitude modulator 9, and a power amplifier means Ill which represents one or more amplifiers for raising the level of the signal to be supplied to the antenna.

To impress a distorting signal upon the rst frequency modulator 5, a source, II, of oscillations adjustable over a suitable range is provided whh is connected to an amplifying and harmo c generating device I2, connected in turn through an amplifier I3 to theV modulator 5. To impress upon frequency modulator 1 the desired signals to be transmitted, the latter'modul'at'oris connected through an amplifier I4 to a source (not shown) of thesedesired signals.

reference to the fo-llowing description quency amplifier 34,

To provide a synchronizing signal for the transmitter I and the receiver 2, oscillator II is connected to the amplitude modulator 9.

To prevent interception of the distorting signals from the transmitter when the desired signals are not being radiated, an interlock I5, described more fully hereinafter, is connected between. the amplifier I4 of the desired signals and the amplifier I3 of the distorting signals.

The frequency modulators 5 and 1 may be of any suitable construction in accordance with well-known principles.

The amplifyingY and harmonic generating device IZ may be constructed in Various ways. For example, device I2 may include doublers, triplers, and mixing stages, (all not shown), of such form and so arranged that any desired harmonic of the frequency supplied by the oscillator II may be obtained. In harmonic generating device I2, the fundamental and the second to the seventh harmonicsthereoi` are obtained respectively through corresponding control means I6 to 22 each comprising. a means, as indicated by the arrows, for adjustment of magnitude of the harmonic, and a switch means, such as the pairs of sockets 23 to 29 adapted to cooperate with plugs 30 to 32, to connect in series` various of the harmonic generating means of device I2 to the amplier I3. The device I2, with the associated control means I6 to 22, may thus be employed to obtain desired wave shapes by adding together various components of the wave supplied by oscillator II, having different magnitudes.

In the receiver 2 of Fig. l` the initial stages thereof which include an antenna 33, a radio frean oscillator 35, a converter 36, and an intermediate frequency amplifier 31, may be of a` usual superheterodyne receiver type. .The output of the intermediate frequency amplifier 31 is connected to two channels 38 and 39.

In the channel 38 the output of intermediate frequency amplifierv 31 is impressed upon a rectifier 40 which is connected through a filter 4I to an amplifying and harmonic generating device; 42. The device 42 is similar in all respects to the device I2 of the transmitter I, having similar amplitude control means, as 43, 44 and 45, and similar switch means, as plugs and sockets 46 and 41, 48 and 49, and 59 and 5I. As in the device I2, in device 42 any combination of the fundamental and the second to the seventh harmonic maybe selected bythe switch means.

In the other channel, 39, of receiver 2, the outputv of intermediate frequency amplifier 31 is impressed upon a converter 52. The output of the amplifying and harmonic generating device 42 is An amplifier.

for decoding of messages at the receiver. The operation of this synchronizing signal Will be explained more fully hereinafter. The complete signal from amplitude modulator 9 is impressed through power amplifier IE upon the antenna 4 and radiated therefrom.

During periods when no speech or other desired signals are being sent from the signal source throughamplifier I4, only the distorting signal comprising the carrier which is frequency-modulated by the complex wave form would be radiated yfrom antenna 4 in the absence yo' preventive lustrated in Fig. l, a carrier wave, the frequency of which, for purposes of illustration, may be in the 4I) to 80 megacycle range,'ls supplied toire-V quency modulator 5 of transmitter I from oscillator 3. A Wave, the frequency of which may be in the 500 to 5900 cycle range, is supplied to am` pliiier and harmonic generator I2 from oscillatorv II. A distorting signal is supplied to frequency modulator 5 through amplifierl3 from device I2. The distorting signal in the illustrated case is constituted by a combination of the fundamental supplied by oscillator II and selected from device I2 by plug 3i and socket 23 of control means I 6, the third harmonic of this fundamental selected by plug and socket 3| and 25 of control means I8, and the fifth harmonic of the fundamental, selected by plug and socket 32 and 21 of control means 20. f

The carrier Wave from oscillator 3, frequency modulated in modulator 5 and having impressed thereon the distortingsignal from device I2, is then impressed upon the second frequency modulator 1 through amplifier 6. A desired signal from the signal source (not shown) is impressed upon the second frequency modulator 'I through the amplifier I4.

The frequency. modulator 1 to which the desired signal is applied from amplier I4 thusreceives, from the amplifier 6, a carrier Wave which has already been frequency modulated, in modulator 5, according to a complex Wave form or code signal supplied through the amplifier I3 andderived from the amplifier and harmonic generator I2. The total distorting signal impressed upon frequency modulator 'I is composed of three independent components, i. e., the average frequency supplied b-y oscillator II, the frequency shift superimposed upon the radio frequency carrier Wave by the frequency modulator 5, and the Wave shape of the distorting signal supplied byA the harmonic generator I2.

Therefore the frequency modulated desired signal which leaves frequency modulator 'I is distorted, and unintelligible if received by yan ordinary frequency modulation or amplitude modulation receiver, signal a large number of combinations of the three independent components may be employed, and in operation of the transmitting station I and the -corresponding receiving station 2 the combination to be used will be determined in accordance with a previously arranged code agreed upon by the operators respectively of the transmitting and receiving stations. v

The distorted signal from modulator 'I is supplied through ampliiier' to amplitude modulator 9. Oscillations of the frequency of oscillator II, or Vharmonics thereof which may be obtained from the harmonic generator I2, are also impressed upon the amplitude modulator 9. The amplitude modulation thus introduced i-nto the signal is preferably small, for example of the order of an'd its purpose is that of a synchronizing signal In selecting the total distortingV means. Therefore, the distorting signal could be Yeasily interceptedand analyzed at a receiving station not provided With the above-mentioned code, and a similar distorting signal could be introduced at the latter station thereby to intercept the transmitter signal when it contained the speech signal. Referring more particularly to Fig. la, to prevent such undesired intercepting of the distorting signal theinterlock I5 is interposedbetween amplifiers I4 and I3. This interlock may comprise a rectifier I5a supplied with current from amplifier I4 and arranged to provide a voltage, across a resistor I5b for example, which voltage normally overcomes a high bias voltage, impressed from a suitable source I5e upon the grid of amplifier I3 and permits the latter to transmit the Wave from device I2. The amplifier I3 is, however, blocked When the speech signal stops, thereby preventing the radiating of the distorting signal from the transmitter in they absence of the speech signal.

The radiated carrier Wave, of 40 megacycle frequency for example, modulated as hereinabove described bya desired signal and a distorting signal, is impressed upon the antenna 33 of receiver2 from the antenna 4.of transmitter I. The superheterodyne initial stages of the receiver 2 may be so arranged that the intermediate frequency from these' stages is, for example, 20 megacycles.

The portion of the 20 megacycle modulated carrier wave impressed from intermediate frequency ing signal, which has been impressed upon the transmitted signal .by the amplitude modulator E of transmitter I. This component, appearing in the output of rectifier 40, is, in the present case, of thesame frequency as that of the output' of oscillator II of Fig. 1. The output of the rectifier 40 is passed through the filter 4I which is so arranged that undesired frequencies are removed leaving the above-mentioned component. The output from the filter is then impressed upon the amplifying and harmonic generating device 42. The fundamentalI frequency and the third and fifth harmonics of the fundamental, which are respectively of the same frequency as the correspending fundamental and third and fifth harmonies from device I2 of Fig. l, are selected from the device 42 by plugs and sockets 45 and 41, 48 ,and 49, and 5U and 5I, and the control means 43, 44 and 45 are so adjusted that the several components are respectively of the same amplitude as the corersponding components from the device I2 of transmitter I.

The ycoding signal Wave, or wave of complex form derived from the device 42 by the combining of the fundamental and its third and fifth harmonics is therefore identical with the coding signal Wave,.or wave of complex form, derived from the device -I2 of the transmitter I. The fundamental frequency, which is 'the Vsame in harmonic, generatorv I2 ofthe transmitter t and in harmonic generator 42 of the receiverv 2, thus serves to.A synchronize: the: frequencies of the coding signal or complex wave at the transmitter I. and' the receiver 2..

The received modulate-dt carrier wave is impressed. from intermediate frequency amplifier 31 upon converter 52 also, in the other branch 39 of the-,receiver 2. The converter 52 performs two. functions. In this converter the frequency of the wave impressed thereon is reduced from the intermediate frequency, in the present case megacycles, toa still lower frequency, for example six megacycles. Further, in the converter 52 the distorting `frequency modulation introduced into the radiated signal by the frequency modulator 5 of the transmitter I is removed. For these purposes the wave of complex form from device 42 is utilized to frequencymodulate the wave having a frequency of 26 megacycles, for example, from oscillator 54, the frequency modulation. of the latter wave: by the wave of complex form being accomplished in the frequency modulator 53 to form a wave including a. demodulating signal wave identical with the frequencyV modulation impressed upon the orlgi. nal carrier wave at modulator 5 of Fig. l. The output of frequency modulator 53 isimpressed upon the converter 52..

In order that the distorting frequency modulation introduced by frequency modulator 5 of the transmitter may be removed, the frequency shift of modulator 53 of the receiver 2l is made equal to` the frequency shift of the corresponding modulator 5 of the transmitter. Then, since the complex wave form from device 42, in ac cordance. with which the output of: oscillator 54 is modulated, is also so selected as to be the same as that of the complex wave form from device I2, in accordance with which the output from oscillator `3 total distortion introduced by the device I2 and frequency modulator 5 of transmitter t is removed in the converter 52.

The output of converter 52 comprises a new carrier wave, of 6 megacycles frequency in the present case, having the. desired signal, in the form of frequency modulation from frequency modulator 1 of transmitter L. superimposed thereon. From this new frequency-modulated carrier wave the desired signal is restoredV by means of the limiter 55 and the frequency modulation-to-amplitude modulation converter 5B, and the signal is then impressed through the audio amplifier 51V upon the indicating device or loudspeaker 58.

Referring to Fig. 2, figure illustrates. a secret signaling system which is based upon and is an extension of the system illustrated in Fig. 1, and which may be found useful in special applications wherein an increased degree of Secrecy over that obtainable by the system of Fig, 1 is desired. For this purpose in the system of Fig. 2 a distorting signal similar to that applied to the carrier wave, as described in connection with Fig. 1, is applied to the desired signal before the latter is impressed upon the carrier wave.. The numerals 59 and 60 designate respectively the transmitter and the receiver ofthe more extended system based on tratedinFlg. 2.

The-parts numbered 3'to. 32 of the transmitter 59-are identical with the. correspondingly numbered-parts in the transmitter of Fig. :l except that in order to supply a second wave of? comis modulated, therefore the Fig. l which is illusl.

plex form in addition to they complex wave supplied to. frequency modulator .5, the. amplifier and harmonic generator I2 in Fig. 2 is; provided with. a. second set of control devices, 6Iv to 61, having a second set of switch means, as sockets 68 to 14, adapted to be connected in any desired combination by connection means including plugs', as 15 and 15:, to a circuit to be presently described.

"liol provide a distorting modulation of thev desired signal before it. is applied at frequency modulator 1 to the. carrier wave, the desired signal i-s impressed, from a. signal source (not shown), through. an amplifier 11 upon a. mixer 1.8 upon which the output of a frequency modulator 19. is also.- impressed through an amplifier 80.. An oscillator 8| producing a wave having a frequency preferably of 10,000. cycles per second is. connected to the latter modulator 19, and a predetermined combination of the. control means of harmonic generator I2 is also connected thereto, as in the present. instance by plugs 15 and 16. In certain cases: the. oscillator 8| may be dispensed with and in place thereof the oscillator [Iv or the harmonic generator I2 may be connected to. the modulator 19.. The mixer 18 is connected through the amplifier I4 to the frequency modulator 1.

It is necessary that a frequency shift which is. relatively very high, of the order of 5,000 cycles` plus or minus per second, be produced in the. frequency modulator 19. The average frequency of this modulator being relatively low, of the order of 10,000 cycles per second or even less, the frequency shift therein must be 50 per cent or more.. Because of the objectionable distortion encountered when the frequency shift extends over such a wide range, the desired. shift is preferably obtained in any desired or known manner which will. avoid this distortion, as by means (not shown) wherein a radio frequency wave, of 1000 kilocycles per second for example, is iirst modulated by waves having frequencies for example ranging up to plus or minus 5000 cycles per second, and then the resulting modulated wave is heterodyned down to the desired average frequency, as 10,000 cycles per second.

To prevent unwanted interception of the distorting signals from the transmitter 59 when the desired signals are not being radiated, the interlock I5 is interposed between amplifier 11 and I3, and an interlock B2 isinterposed between amplifiers 1T and 80..

To provide a synchronizing mitter 59' and the receiver signal for the trans- 60 either the oscillator II or the harmonic generator I2 may be connected, through a switch as 83, to the amplitude modulator 9.

In the receiver 60 ofv Fig. 2 the parts numbered 33 to 31 and 52 to 58 are identical with the correspondingly numbered parts in the receiver 2 of Fig. 1. As in Fig. 1, an amplifier (not shown) may be added between converter 52 and limiter 55. Similarly to Fig. l, the output. of the intermediate frequency amplifier 31' of Fig. 2 iskconnected into two channels, 84 and 85.

In the channel 84 the output of amplifier 31 is impressed, through a rectifier 86 and a filter 81, upon an amplifier andharmonic generator 88 which is identical with the amplifier and harmonic generator I2 of transmitter 59.

In the otherchannel, the output of amplifier 31 is impressed upon the converter 52 as in the receiver 2y of Fig. '1, and the converter 52, frequency modulator' 53, oscillator 54, limiter 55,

and frequency-modulation-to-amplitude modulation converter 58 bear the same relation to each other as in the receiver 2. "Further, similarly vto receiver 2, the control means, for example 89, 90, and 9|, representing a selected series combination of the fundamental and harmonics thereof in device 88 are connected to the frequency modulator 53, for example through the switch means 92 and 93, 94 and 95, and 98 and 91.

The frequency modulation-to-amplitude modulation converter 56 in receiver 60 of Fig. 2 instead of being connected through amplifier 51 to the signal responsive device or loudspeaker 58 as in receiver 2 of Fig. 1, is connected to a mixer'98. A frequency modulator' 99 having a frequency shift equal to that of the corresponding modulator 19 of the transmitter 59 is also connected to the mixer 98. To this frequency modulator 99 are connected an oscillator |00, and also selected control means as IOI and |02 ofthe amplifier and harmonic generator 88 through switch means |03, |04 and I 05, |06, corresponding respectively to control means 62 and 63 and switch means 15, 69 and 16, 10 of amplifier and harmonic generator II2 of transmitter 59. The switch means |03, |04

and |05, |06,l are preferably connected to frequency modulator 99 through a phase shifter |01.

Under certain conditions the oscillator |00 may be dispensed with and the harmonic generator 88 may be connected to the frequency modulator 99 to supply the wave of the required frequency.

Themixer 98Vis connected, through a low pass filter |08 and the amplifier 51, to the signal indieating means or loudspeaker 58.

In operation of the system of Fig. A2, a carrier wave from oscillator 3, frequency ,modulatedV in the modulator 5 in accordance with a distorting signal waveof complex form from the amplifier and harmonic generator I2 through the amplifier I3, is impressed upon the frequency modulator 1 through the amplifier 6. The operation up to this point is thus the same as described in connection with Fig. 1.

A desired signal in the 0 to 5,000 cycles per second frequency band for example, from the signal sourcev (not shown) is impressed through amplifier 11 upon the mixer 18. The wave from oscillator 8| is frequency-modulated in modulator 19 in accordance with a distorting signal from amplifier and harmonic generator I2.. Thelatter signal in the present case is constituted by the second and third harmonics, selected respectively by switch means 15, 69 and 18, 10. The resultant distorting frequency-modulated wave is also impressed, through amplifer 80, upon the mixer 18. The desired signal combinedin mixer 18 with the distorting frequency-modulated wave from modulator 19 is impressed through the amplifier I4 upon the frequency modulatr 1.

The distortion in the frequency modulated desired signal which leaves the frequency modulator 1 of Fig. 2 is thus not only due, as in the transmitter of Fig. 1, to the distorting signal impressed upon the modulator 1 from frequency modulator 5, but also to the distorting signal supplied through the mixer 19 from frequency modulator 19. In selecting the total distorting signal, the combination of harmonics and their amplitudes, from harmonic generator 2, which is impressed upon the frequency modulator 99 is, in general, different from the combination impressed upon the frequency modulator 5. -The purpose is to render the derived audiosignal unintelligible due to voice controlled A masking frequencies. I t is obvious that the well-known: forms of voice scrambling, speech inverting or other such systems may provide additional impediment to non-authorized reception. l v

oscillations from oscillator I I or from harmonic generator I2 are impressed through switch 83 upon the amplitude modulator 9 to produce a synchronizing signal. Undesired interception-of the distorting signal when the desired signal :is not being transmitted is prevented by interlocks I5 and 92 which operate in the manner previously explained in connection with Fig. l.

The distorted signal from modulator] is impressed, as in Fig. 1, through amplifier 8, amplitude modulators 9 and power amplifier I0, upon antenna 4 and radiated therefrom. v

In receiving the desired signal from the' trans mitter 59 by the receiver 60, 'the operation is sub.. stantially the same as that described in connection with the operation of receiver 2 of Fig. 1 up to the output side of frequency modulation-toamplitude modulation converter 56. In the' system of' Fig. 2, however, the wave in the output of the converter 56 comprises, instead of the 6 megacycle carrier wave modulated only by the desired signal, a 6 megacycle carrier wave modulated by the desired signal which itself has previously been modulated inaccordance with a distorting signal from frequency modulator 19 of transmitter 59.

AThe output of the frequency modulator 99, which includes a demodulating signal wave similar to the distorting frequency-modulated wave u from frequency-modulator 19 of Fig.- l, enters the mixer and detector 98, to which the output of the converter 56 is also supplied. The voutput of mixer and detector 98 is impressed upon the low-pass filter |08 which is arranged, in thel present case, to pass the'O to 5,000 cyclesper 'second frequency band. The desired signal, comprised in this band, is present in the output of mixer 98, being supplied to the mixer from converter 58. The output of filter IIIB'therefore represents only the desired signal, which is in this band. The restored desired signal after amplification in the amplifier 51 is supplied to the signal indicating device or loudspeaker 58. y

' It will be understood that in' the secrecy signalling systemsdescribed herein additional degrees of secrecy may be obtained by applying known secrecy systems to the input signal., For example, inverted speech methods, speech scrambling, and methods involving the mixing and inversion of certain frequency bands, etc., as well-known in telephone practice, may be employed.

My invention has been described herein in particular embodiments for purposes of illustration. It is to be understood, however, that the inven tion is susceptible of various changes and modifications and thatv by the appended claims I intend to cover any such modifications as fall within the4 true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a secret signaling system, a transmitter comprising means to transmit a carrier wave and to modulate said carrier wave withA a'desired signal and a masking signal, said masking signal having harmonically related components, means to select said harmonically related components in accord with a prearranged code, areceiver for said carrier wave including means to generate in said receiver currents having any frequency comprised in said harmonically` related components,4

means in said receiver to select said frequencies invaccord with said code, Ymeans'utilizing'the-frequencies selected in said recei' 'er to suppress said masking signal as received in said receiver from said transmitter, means to transmit a synchronizing current of desired frequency between said transmitterand receiver, and means to synchronize the frequencies of "said components of said masking signal and the frequencies corresponding thereto generated in said receiver by the frequency of said synchronizing current.

2. In combination, a transmitting station and a receiving station, means to transmit from said transmitting station a carrier wave modulated in accord with a desired signal, means in said receiving station to reproduce said signal from said carrier wave, and means to prevent undesired interception of said signal, said last means comprising means in each station to generate currents having a plurality of harmonically related frequencies and to select in each station one or any desired combination of said frequencies in accord with a prearranged code, means to transmit synchronizing current of desired frequency between said stations and to synchronize therewith all of said harmonically related currents in both stations, means in the transmitting station to modulate the frequency of said carrier wave in accord with currents having the selected frequency or frequencies, and means in said receiver utilizing the one or more of said harmonically d related frequencies selected therein to remove said frequency modulation from said carrier wave.

3. The method of signaling secretly between a transmitting station and a receiving station which comprises modulating a carrier wave at the transmitting station with the signals to be transmitted and modulating the frequency of said carrier wave in accord with any one or more of a plurality of harmonically related frequencies,

generating in said receiver currents having said .l

harmonically related frequencies, synchronizing said harmonically related frequencies in both of said stations with a single frequency transmitted between said stations, and selecting said one or more of said frequencies in each station in accord with a prearranged code, receiving said frequency modulated carrier wave in said receiver and heterodyning it with oscillations having frequency varying in accord with said one or more of said harmonically related frequencies generated in said receiver thereby to produce a carrier wave free of modulation in accord with said one or more harmonically related currents, and reproducing the desired signal from said last mentioned carrier wave.

4. In a signaling system, a transmitter comprising means to frequency modulate a carrier wave in accordance with a distorting signal Wave, means to frequency modulate said carrier wave in accordance with a desired signal wave, and means responsive to said carrier wave modulation in accordance with said desired signal to prevent said carrier wave modulation in accordance with said distorting signal wave in the absence of said carrier wave modulation in accordance with said desired signal.

5. In a signaling system, a transmitter, a receiver, means in said transmitter to frequency modulate in succession a carrier wave in accordance with a distorting signal wave and in accordance with a desired signal wave, means in said receiver to frequency modulate a second carrier wave in accordance with a distorting signal wave identical with said first-named distorting signal wave, means operatively associated with said transmitter andsaid receiver to svncn1-onizesaid lastname'd distorting signal wave .with said first-named distorting signal wave, and means in said receiverzincluding said second-named means to derivesaid desiredsi'gnal wave from said lirstnamed carrier wave.

`6.. In' a signaling..system 'a transmitter comprising means to frequency modulate a carrier wave at a predetermined frequency shift inaccordance with a distorting signal wave, means to frequency modulate said carrier Wave in accordance with a desired signal Wave, and means to amplitude modulate said carrier wave in accordance with a predetermined synchronizing signal wave, and a receiver comprising means to derive from said synchronizing signal wave a distorting signal wave identical with said first-named distorting signal wave, means to frequency modulate a second carrier wave at said predetermined frequency shift in accordance with said lastnamed distorting signal wave, and means including said lastenamed means to derive said desired signal wave from said first-named carrier Wave.

7. In a signaling system, a transmitter comprising means to frequency modulate a carrier Wave in succession in accordance with a distorting signal wave and in accord-ance with a desired signal wave, and means to amplitude modulate said carrier wave in accordance with a synchronizing signal wave for decoding of the resultant signal transmitted from said transmitter.

8. In a signaling system, a tr-ansmitter comprising means to frequency modulate a carrier wave at a predetermined frequency shift in accordance with a distorting signal Wave, means to frequency modulate said carrier Wave further in accordance with a desired signal Wave, and means to amplitude modulate said carrier wave in accordance with a predetermined synchronizing signal Wave for decoding of the resultant signal transmitted from said transmitter.

9. In a signaling system, a transmitter comprising means to frequency modulate a carrier wave in accordance with a distorting signal wave, means to frequency modulate said carrier Wave in accordance with a desired signal wave, means to prevent said carrier wave modul-ation in accordance with said distorting signal Wave in the absence of said modulation in accordance with a desired signal wave, and means to amplitude modulate said carrier Wave in accordance with a synchronizing signal wave for decoding of the resultant signal transmitted from said transmitter.

l0. In a signaling system, a receiver comprising means to receive a carrier Wave frequency modulated in succession by a distorting signal Wave and by a desired signal Wave and amplitude modulated by a synchronizing signal wave, and means to derive from the synchronizing signal Wave modulation of said received carrier Wave a distorting signal wave identical with said firstnamed distorting signal wave.

ll. In a signaling system,l a receiver comprising means to receive a carrier wave frequency modulated in succession by a distorting signal Wave and by a desired signal wave and amplitude modulated by a synchronizing signal wave, means to derive from the synchronizing signal modulation component of said received carrier wave a distorting signal wave identical with said ilrstnamed distorting signal wave, means to frequency modulate a second carrier wave in accordance with said last-named distorting signal wave, and means including said last-named frequency modulating means to derive said desired wave, and a receiver adapted to receive said can:- signal wave from said first-named carrier wave. rier wave, said receiver comprising means to gen- '12. In a signaling system, a transmitter comerate a Waveidenticalwith said distorting signal prising means to frequency modulate a carrier Wave and means'includingy said /iastfnam'ed Wave in accordance with a distorting signal wave 5 means to remove from'said carrier wave'said dis-i and means to frequency modulate the resulting torting frequency modulationV thereof; i c carrier wave in accordance with a desired signal ELLSWORTH D.LC,OOK.' 

